Smaller Friction Coefficient Research Articles

Experimental investigation on Stribeck curves of different elastic modulus materials under oil and water lubrication conditions

Purpose The purpose of this paper is to study the influence of friction coefficient of materials with different elastic modulus on the variation of velocity and load under water lubrication and oil lubrication conditions. Design/methodology/approach Low-viscosity lubricating oil and water were used as lubricants to test the friction performance of the ball-disc contact friction pair in the lubrication state on the universal micro-tribometer multi-functional friction and wear test system. Findings In the same speed range, the lubrication states from soft to rigid materials are not necessarily similar to each other. Generally, the material with low elastic modulus is suitable in low-viscosity lubricant environments, while the material with high elastic modulus has relatively smaller friction coefficients in oil-lubricated environments compared with water lubrication. However, the coefficients of polyethylene, polytetrafluoroethylen and polyoxymethylene are exceeded by rubber’s coefficients under water lubrication in the same experiment environments, and their lubrication states are not affected by lubricants. The friction coefficient of the friction pair decreases with the increase of loads; however, it does not apply to all materials. The friction coefficients of materials with smaller elastic modulus such as rubber under high loads are rather large. Therefore, the elastic modulus of the material under high loads is a factor to be considered. Originality/value The Stribeck curves study of the ball-disk contact friction pair comprising soft and rigid materials, whose elastic modulus is from hundreds of GPa to a few of MPa, was carried out. The influence of different speeds, loads and lubricants on the friction coefficient of the friction pair was revealed, which provided a research basis for the selection and matching of friction pair materials.

Physic-chemical analysis for high-temperature tribology of WC-6Co against Ti–6Al–4V by pin-on-disc method

To study high-temperature tribological behaviors between tools and workpieces plays an active role in the development of high-speed machining. This original work dedicated to investigate high-temperature tribological behaviors of Ti–6Al–4V/WC-6Co interaction couples at the temperature ranging 20 °C, 320 °C, 620 °C, and 920 °C, which analyzed the physic-chemical properties of coefficient of friction, tribological topography, element diffusion, phase compositions, and nano-indentation hardness. Smaller coefficient of friction was existed under the high-temperature due to softer Ti–6Al–4V serving as lubricating layers at the interface. The tribological behaviors were gradually changed by the temperature conditions, which converted mechanical into the coupled mechanical-chemical induced wear. The effects of high-temperature conditions on WC-6Co could result in a certain degree of nano-indentation hardness weakening. This work provides an in-depth understanding of tribological behaviors and failure mechanism of tool materials in machining Ti–6Al–4V.

Study of the effect of surface laser texture on tribological properties of cemented carbide materials

Surface texturing has become a potential method to obtain a low friction coefficient under dry/lubricated conditions for different mechanical product surfaces. The mechanism of friction and wear reduction from a micro-texture on the surface of cemented carbide cutting tools was investigated by dry cutting a titanium alloy. Three kinds of micro-textures, namely, line, sinusoidal and rhombic grooves, with different area occupancy rates were produced by a laser on the cemented carbide surface. Experiments and finite element simulation of ABAQUS were used to investigate the tribological characteristics of micro-textured cemented carbide. The results indicated that the line-textured cemented carbide with an area occupancy rate of 10% showed a low friction coefficient of 0.076, which is approximately 34% of the non-textured sample. Few adhesives appeared on the textured surface, while a large number of adhesives were attached to the smooth surface after 30 min of dry friction between the cemented carbide and the titanium alloy balls. Moreover, among the three textures, the line-groove texture has the smallest friction coefficient and a good anti-wear effect. The results show that the existence of a groove texture can effectively reserve the wear debris, reduce the bond wear and weaken the furrow effect.

Grinding performance and tribological behavior in solid lubricant assisted grinding of glass-ceramics

High dimensional stability of glass-ceramics is highly attractive to optical and precision engineering industries. This study aims to provide an efficient and stable grinding strategy for realizing high-quality surface and dimensional accuracy of glass-ceramics. The different solid additives (CeO2 and SiC) as lubricants and their performances were investigated regarding the grinding forces, friction coefficient, wheel wear and surface finish during grinding glass-ceramics. Friction and wear mechanism in solid lubricant assisted (SLA) grinding were analyzed. Three-body abrasive wear and chemical mechanical interaction were considered as the predominant lubrication mechanisms. The results showed that the low, stable grinding force and small friction coefficient can be obtained under SLA-SiC grinding. The study proved that the novel strategy can keep the wheel sharpness and improve the process performance.

Biomimetic Anti-Adhesive Surface Microstructures on Electrosurgical Blade Fabricated by Long-Pulse Laser Inspired by Pangolin Scales.

The electrosurgical blade is the most common invasive surgical instrument in a cutting and hemostasis process; however, the blade easily leads to the adhesion of overheated soft tissues on the blades and induces a potential danger for the patients. To minimize the adhesive tissues, we proposed the one-step surface texturing method to fabricate anti-adhesive biomimetic scales on stainless steel 316L rapidly based on the self-organized surface microstructures induced by the long-pulse fiber laser, which was inspired by the excellent performances of anti-adhesion and anti-friction in the pangolin scales. The optimal formation parameters, chemical components, and crystal structures of the laser-induced self-organized surface microstructures were investigated in the experiments. Moreover, the underlying formation mechanism was revealed. The electrosurgical blades with biomimetic scales have hydrophobicity and a smaller frictional coefficient, which effectively reduced the adhesion of soft tissue.

On Hopf bifurcation in the problem of motion of a heavy particle on a rotating sphere: the viscous friction case

We investigate the Hopf bifurcation of a mass on a rotating sphere under the influence of gravity and viscous friction. After determining the equilibria, we study their stability and calculate the first Lyapunov coefficient to determine the post-critical behavior. It is found that the bifurcating periodic branches are initially stable. For several inclination angles of the sphere’s rotation axis, the periodic solutions are calculated numerically, which shows that for large inclination angles turning points occur, at which the periodic solutions become unstable. We also investigate the limiting case of small friction coefficients, when the mass moves close to the equator of the rotating sphere.

Numerical prediction of interaction between turbulence structures and vortex cavitation

ABSTRACTA novel numerical method to perform direct numerical simulations (DNS) of turbulent flow was established to predict the characteristics of compressible gas–liquid two-phase flow. A cavitation model was introduced to reproduce the phase change in the cavitation phenomena, and a DNS of wall turbulence of water with cavitation was performed. Vortex cavitation was observed in the wall turbulence. The growing cavity in the core of the vortex changed the vortex intensity. After vortex cavitation finished, the friction coefficient for the cavitating flow became lower than that of non-cavitating flow. The variation tendency of turbulent statistics corresponded to that of experimental measurements. For a small friction coefficient of the cavitating flow, the redistribution term in the streamwise component of the turbulent kinetic energy equation was reduced owing to the vortex cavitation. Consequently, it was proved that the occurrence of vortex cavitation modified the redistribution term, resulting in turbulence modification.

Effects of h-BN particles on the microstructure and tribological property of self-lubrication CBN abrasive composites

Hexagonal boron nitride (h-BN) particles as solid lubricants were used in metallic cubic boron nitride (CBN) abrasive composites to reduce severe frictions at the tool–workpiece interaction. The microstructural characteristics and mechanical properties of sintered samples with different concentrations of h-BN particles ranging from 5 wt% to 15 wt% were evaluated. Morphological characterisation of the wear plat of balls and scratch surface were performed after the rotating sliding test. Results showed that the micropores and flexural strength rapidly increase as h-BN concentrations increase. The fracture evolution characteristics from trans-granular to inter-granular of CBN abrasives were observed. Samples with high h-BN concentrations possessed small friction coefficients and poor wear-resistance capability.

Two-Body and Three-Body Wear Behavior of a Dental Fluorapatite Glass-Ceramic

As a veneering porcelain coating of dental prosthesis, two-body and three-body wear behavior of dental glass-ceramic with the main crystalline phase of fluorapatite has not been comprehensively studied. In this work, a self-made fluorapatite glass-ceramic was synthesized and the mechanical and tribological performances of the glass-ceramic were tested, comparing with a commercial feldspathic glass-ceramic. The friction and wear experiments were performed between disk-shaped glass-ceramics and natural teeth in two-body (dry, water, saliva) and three-body (slurry) modes, respectively. Results showed that good mechanical properties of fluorapatite glass-ceramic can be achieved by the sintering process. In both two-body and three-body modes, the fluorapatite glass-ceramic had a smaller friction coefficient and wear rate and caused less damage on antagonistic teeth than the feldspathic glass-ceramic. The greater mechanical properties give fluorapatite glass-ceramic a better wear resistance and reduce the adhesive wear.

The effects of applied voltage on surface texturing during cathodic plasma electrolysis process

Cathodic plasma electrolysis (CPE) process was applied on cast iron samples for about 1 minute to obtain a crater-liked surface texture with intention to reduce the friction and increase the wear resistance. During the treating process, the plasma discharging was initiated at the surface of cast iron samples, leading to the explosion of gas bubbles and then generating an irregular array of micro craters. The scanning electron microscopy (SEM) observations showed that the recessed and protruded surface textures were obtained when the CPE-process was conducted at low and high voltages, respectively. The textured surfaces were measured and characterized using skewness and kurtosis Pin-on-disc tribotests on those textured samples demonstrated that the samples with negative skewness and higher kurtosis had a smaller coefficient of friction (∼0.08), while the samples with positive skewness and higher kurtosis had a larger coefficient of friction (∼0.104), which was even higher than that of untreated blank sample (∼0.1). The results indicate that the applied voltage significantly influences the surface finish in terms of surface texture and morphology, and thus the coefficients of friction.

Motion of a Puck on a Rotating Horizontal Plane

The motion of a puck on a horizontal plane rotating about a vertical axis with dry friction is considered. It is assumed that the Coulomb law of dry friction is locally valid at each point belonging to the lower surface of the puck. The resultant force and friction torque are determined according to the dynamically consistent model of contact stresses. This problem generalizes the problem of motion of a puck on a fixed plane and the motion of a disk (a puck of zero height) on a rotating plane. The invariant sets of the problem are found and their properties are studied. In the case of a sufficiently small Coulomb friction coefficient, the general solution to the equations of motion of the puck is constructed as a power series expansion with respect to this coefficient.

THE EFFECT OF SURFACE ROUGHNESS AND MATERIAL HARDNESS ON THE TRIBOLOGICAL PERFORMANCE OF THE SLIDING PAIR WC-42CrMo4 UNDER STARVED LUBRICATION

Ceramic materials are more and more popular in industrial applications. Machine elements made of ceramic materials are characterized by long durability, especially those elements that work in hard operating conditions. In this paper, the results of tribological examinations of WC-42CrMo4 sliding pair are presented. Tribological tests were realized with the T-11 stand with the ball-on-disc sliding pair. For lubrication, the machine mineral oil L-AN 46 was used. The analysis showed that surface roughness significantly affected the tribological performance of tested sliding pairs. Steel samples were characterized with various hardnesses in the range of 22-42 HRC and this feature also effected on friction coefficient. Statistical analyses of the achieved results were made using the 3-level PS/DC 32 design. According to the statistical analysis, the quadratic functions describing the dependence for friction coefficient on surface roughness and material hardness were obtained. The most unfavourable surface roughness was after the grinding process, but the best was those after the lapping process which allows the smallest friction coefficient.

Effect of plasma electrolytic oxidation process on surface characteristics and tribological behavior

Plasma electrolytic oxidation (PEO) produces oxide ceramic coatings on lightweight Al alloys with high hardness, low thermal conductivity and dimple-like surface morphology. By changing PEO process parameters, coating surface morphology can be manipulated for friction reduction. This study uses both unipolar and bipolar current modes to prepare oxide coatings on Al-Si alloy samples with different durations in order to obtain surface textures with different dimple sizes and densities. The results indicate that decreasing actual contact areas of the coating surfaces and their counterparts lowers the coefficient of friction. The real contact areas, which can be observed in the frequency density distribution histograms and curves of the surface profile data, were different initially and then varied during the sliding tests in progress. More negatively and downwardly shifted maximum values on the probability density distribution function curve of a coating surface profile indicate less real contact area and thus a smaller coefficient of friction. Hardness also plays a factor in frictional behavior.

Tribological properties of the PA46 and Teflon added PA46 polyamides

In order to increase the efficiency of the mechanical transmissions, one of the actually research directions is oriented to develop new materials characterised by high endurance, small friction coefficients and internal structure stability with the temperature. One of these materials is represented by the polyamides which are used in mechanical industry, food industry and medicine. The paper provides the results of tests performed on a tribometer for a PA46 and a Teflon added PA46 polyamide materials being on contact with a PA46 polyamide disk, in lubricated conditions. The tests are achieved for a set of temperatures of the oil bath, a set of rotational speeds of the PA46 polyamide disk and a continuous variation of the local pressure. During the running-in process there are measured the evolution of the dynamic friction coefficient and of the wear. After this, it is studied the variation of the dynamic friction coefficient with the contact pressure, the rotational speed and the temperature. Finally there are given conclusions regarding the tribological behaviour of the tested materials and regarding their applications.

Tribological properties of TiN-Pb system solid lubricant coatings with various morphologies

The paper presents the results of the investigation of the solid lubricant coatings (SLC) tribological properties of several designs based on a TiN matrix with the addition of Pb (∼6% atomic). A comparison of the tribological properties is made between the TiN coating without the addition of a solid lubricant component, a monolayer coating of TiPbN with a TiPb adhesive layer and a multilayer coating with alternating TiPb / TiPbN layers. It is shown that the monolayer coating has the smallest friction coefficient (∼0.2), while the friction coefficient of a multilayer coating is ∼0.75, which coincides with the friction coefficient for the TiN coating. Despite the friction coefficient high level of the multilayer coating, its resistance to wear is at the level of the monolayer coating.

Influencing factors analysis on coiled tubing-in-riser systems considering the force excitation of ocean loads

The mechanical behavior of coiled tubing-in-riser systems is critical to success of coiled tubing operations in the development of offshore oil and gas. The purpose of this work is to guide the coiled tubing's marine applications by analyzing influencing factors of the coupled system. This paper describes a mechanical model of micro elements of the coiled tubing-in-riser system under force excitation of ocean loads. Nonlinear equations simplified by finite difference method are solved through the Newton-Raphson iteration method. Then the analytical analysis is compared with a three-dimensional finite element model of the coiled tubing-in-riser systems. To ensure safe operation, lateral displacement and internal bending moment of the riser are calculated. Besides, four influencing factors annular clearance, ocean depth, platform offset, and friction coefficient are investigated. The results show that strength design of the riser needs to consider the critical position, and larger outer diameter of coiled tubing, smaller water depth operation, smaller platform offset, and smaller friction coefficient are preferred to improve the injection efficiency of coiled tubing. This research can provide important theoretical support for ocean applications of coiled tubing.

Influences of TiAlN Coating on Cutting Temperature during Orthogonal Machining H13 Hardened Steel

TiAlN has been widely used in cutting tool coating due to its excellent mechanical and thermal performances. However, the research on the TiAlN coating effect on cutting temperature is not comprehensive enough. In this paper, the friction tests at elevated temperature and hard machining H13 hardened steel tests are conducted by using TiAlN coated tools and uncoated tools, respectively. The results of using TiAlN coated tools are compared with those from using uncoated tools. It is found that the coefficient of friction (COF) between TiAlN coated tool and H13 hardened steel is reduced to 0.63 at 800 °C. The COF value is 0.75 for uncoated tool. Under the same cutting conditions, the TiAlN coating shortens tool-chip contact length. The tangential cutting forces and cutting zone temperatures are decreased with smaller COF and shorter tool-chip contact length. Due to the lower thermal conductivities of TiAlN coating and the Al2O3 oxide layer formatted at tool rake face, the cutting heat conducted into cutting tool substrate was reduced. The cutting temperatures in TiAlN coated tool substrate are decreased by at least 10.68% in this study. The TiAlN coating reduces the cutting temperature by decreasing the cutting heat generation and conduction.

Tribological performances of graphite–MoS2 coating at various high temperatures

The tribological performances of the graphite–MoS2 coating deposited on the disc specimen, whose material is GCr15 steel, are experimentally investigated under high temperatures. Effects of graphite content, applied load, rotational speed and test temperature on the coefficient of friction and wear scar width of the coating are evaluated. The graphite–MoS2 coating with 20 wt% graphite addition exhibits the relatively small coefficient of friction and wear scar width at 200 ℃. At or beyond 400 ℃, the coating's lubricating effect is severely deteriorated with the testing time. At 200 ℃, there exists a critical applied load of 10 N and a critical rotational speed of 1250 rpm beyond which the MoS2 coating with 20 wt% graphite content is worn through and brings out a large coefficient of friction. The test results indicate that adding 20 wt% to 33 wt% graphite to MoS2 coating leads to an obvious reduction of the coefficient of friction and wear scar width for the coating at 200 ℃.

Heat-Transfer Characteristics of Liquid Sodium in a Solar Receiver Tube with a Nonuniform Heat Flux

This paper presents a numerical simulation on the heat transfer of liquid sodium in a solar receiver tube, as the liquid sodium is a promising heat-transfer candidate for the next generation solar-power-tower (SPT) system. A comparison between three mediums—solar salt, Hitec and liquid sodium—is presented under uniform and nonuniform heat-flux configurations. We studied the effects of mass flow rate (Qm), inlet temperature (Tin), and maximum heat flux (qomax), on the average heat-transfer coefficient (h) and the friction coefficient (f) of the three mediums. The results show that the h of liquid sodium is about 2.5 to 5 times than other two molten salts when Tin is varying from 550 to 800 K, Qm is 1.0 kg/s, and qomax is 0.1 MW/m2. For maximum heat fluxes from 0.1 to 0.3 MW/m2, the h of liquid sodium is always an order of magnitude larger than that of Hitec and Solar-Salt (S-S), while maintaining a small friction coefficient.

ENERGY LOSS AND WEAR IN SPHERICAL OBLIQUE ELASTIC IMPACTS

Percussive and erosive wear by repetitive impacting of solid particles damages surfaces even at low impact velocities. As the impact wear is often directly related to the energy loss during the collision and therefore to the coefficients of normal and tangential restitution, in the present study the oblique low-velocity impact of a rigid sphere onto an elastic half-space is analyzed based on the known respective contact-impact solution and with regard to the energy loss during the impact. Simple analytic expressions are derived for the total impact wear volume. It is found that the portion of kinetic energy lost in frictional dissipation has a well-located maximum for configurations with weak forward pre-spin. The distribution of frictional dissipation over the contact area has a complex dependence on the impact parameters. For pronounced local slip (e.g. due to a small coefficient of friction) the dissipation accumulated over the collision is localized in the center of impact whereas for dominance of sticking, most energy is lost away from the center.